Device and method for generating and decoding a side channel signal transmitted with a main channel signal

ABSTRACT

For generating a signal to be transmitted original information is encoded into a main channel and a side channel, wherein the side channel is more robust against channel influences than the main channel. On the receiver side, when the receive quality is above a threshold, which is necessitated to execute a successful decoding of the main channel, the main channel is reproduced. If the receive quality falls below this threshold, however, the side channel is reproduced which may have less bits than the main channel and which is a correspondingly lower quality representation of the original information than the main channel.

BACKGROUND OF THE INVENTION

The present invention relates to information encoders which may be usedfor broadcasting systems, and in particular to information encoders usedin digital broadcasting systems.

Digital audio broadcast is continually developing. For some time, DAB(digital audio broadcast) has existed for USW frequencies, and for ashort time also DRM (digital radio mondial) has existed for long, mediumor short waves.

Such broadcasting systems distinguish themselves by a certain number ofdata which have to be buffered before the broadcast or radio receiver,respectively, can start data output. When the radio listener switches onhis radio receiver, this is not very problematic as the radio receiversimply starts reproducing a little later, and this interval is notperceived by the radio listener as especially disturbing. When the radiolistener changes the program, however, the listener is used, fromprevious systems, to data being output immediately. With a digitalbroadcasting receiver this is, however, not possible as a certain amountof data has first of all to be stored again by the other program beforea reproduction of this other program can be started. This delay will beperceived by the user as disturbing as it does not occur when he startslistening to the radio but while he is listening.

U.S. Pat. No. 6,842,724 B1 disclosed a method and a device for reducingthe initial delay in data packet-based streaming applications or in atelecommunication network, like, for example, a local exchange carriernetwork or an inter-exchange carrier network or a local or globalcomputer network. A program source, for example an audio and/or videodata stream, is encoded and transmitted as two or more separate bitstreams, for example sequences of data packets. The transmission of oneof these bit streams is delayed by a given delay with regard to thetransmission of the other bit stream. At the receive end of thetransmission channel, the two or more bit streams are buffered byreceiver buffers of different sizes, from which different time delaysresult when the content of the buffers is decoded. In particular, thetime delay difference (inverse) corresponds to the relative delay timesbefore transmission. For encoding, either a rewritable source encodingscheme or an embedded encoding scheme is used, wherein at least one ofthe individual bit streams is sufficient to generate a satisfyingdecoded signal, wherein adding the other bit stream will improve thequality of the decoded signal. Alternatively, the data streams may beseveral encoded representations of the program source having differentbit rates, wherein encoded representations with a lower bit rate aretransmitted with correspondingly greater delays.

In contrast to the transmission types described in the above-mentioneddocument, in which only the delay of a data package has to beconsidered, with digital broadcasting systems further challenges occurwhich result from the fact that the transmission channel is not aline-connected channel but a wireless transmission channel. Atransmitter thus comprises a transmit antenna which emits radio waveswhich may be received and processed by a receiver comprising a receiveantenna. Due to the fact that the transmission channel varies, which maybe due to the system, and which may also result from the fact that thetransmitter and especially the receiver is moving, a transmissionchannel may become better or worse and, in particular, so bad that theconnection from the receiver to the transmitter is abruptly interrupted.In particular, as is generally the case with digital transmission, theconnection below an SNR given by the system will simply be interruptedand there will be no so-called “graceful degradation”, i.e. adegradation of transmission quality taking place gradually, as occurswith analog radio broadcasting in an agreeable way. The interruption ofthe connection below an SNR given by the system (SNR=signal noise ratio)is a problem in particular with DRM, as DRM use short wave carrierswhich have a slowly varying SNR, which results in repeated failures.

With DAB, this threshold of receive quality and/or the SNR at which theconnection is interrupted is problematic, as aligning the antenna orfinding a good receive position becomes more difficult than in analogbroadcasting and becomes even more complex in particular when thereceiver is moving.

SUMMARY

According to an embodiment, a device for generating a signal to betransmitted may have an encoder for generating an encoded signal from aninformation signal, wherein the encoder is implemented to generate amain channel and a side channel which may be decoded separate from eachother and represent the information signal, and to generate the mainchannel and the side channel so that the side channel is more robustagainst transmission-channel influences than the main channel.

According to another embodiment, a device for generating a decodedsignal may have a receiver for receiving a receive signal having a mainchannel and a side channel which may be decoded separate from eachother, wherein a minimal signal/noise ratio for decoding the sidechannel is smaller than a minimal signal/noise ratio for decoding themain channel; a decoder for generating a main channel which is separatefrom the side channel; a quality observer for assessing a receivequality; and a changeover switch which is controllable by the qualityobserver to provide the side channel as a decoded signal when thereceive quality is lower than a threshold quality and to provide themain channel as a decoded signal when the receive quality is higher thanor equal to the threshold quality.

According to another embodiment, a method for generating a signal to betransmitted may have the steps of generating an encoded signal from aninformation signal, wherein the encoder is implemented, by generating amain channel and a side channel which may be decoded separate from eachother and represent the information signal, and wherein the main channeland the side channel are generated such that the side channel is morerobust against transmission-channel influences than the main channel.

According to another embodiment, a method for generating a decodedsignal may have the steps of receiving a receive signal having a mainchannel and a side channel which may be decoded separate from eachother, wherein a minimal signal/noise ratio for decoding the sidechannel is smaller than a minimal signal/noise ratio for decoding themain channel; generating a main channel separate from the side channel;assessing a receive quality; and providing, as a decoded signal, theside channel, when the receive quality is lower than a thresholdquality, or providing, as a decoded signal, the main channel, when thereceive quality is higher than or equal to the threshold quality.

According to another embodiment, a computer program may have a programcode for executing the method of generating a signal to be transmitted,which may have the steps of generating an encoded signal from aninformation signal, wherein the encoder is implemented, by generating amain channel and a side channel which may be decoded separate from eachother and represent the information signal, and wherein the main channeland the side channel are generated such that the side channel is morerobust against transmission-channel influences than the main channel,when the computer program runs on a computer.

According to another embodiment, a computer program may have a programcode for executing the method of generating a decoded signal which mayhave the steps of receiving a receive signal having a main channel and aside channel which may be decoded separate from each other, wherein aminimal signal/noise ratio for decoding the side channel is smaller thana minimal signal/noise ratio for decoding the main channel; generating amain channel separate from the side channel; assessing a receivequality; and providing, as a decoded signal, the side channel, when thereceive quality is lower than a threshold quality, or providing, as adecoded signal, the main channel, when the receive quality is higherthan or equal to the threshold quality, when the computer program runson a computer.

An embodiment may have an encoded signal having a main channel and aside channel, wherein the side channel is formed such that it is morerobust against transmission-channel influences than the main channel.

The present invention is based on the finding that a reduction ofunpleasant data failures may be achieved when the one-channel idea isnot considered, in which the program material is encoded andtransmitted. According to the invention, in addition to the typicallytransmitted program material representing the main channel, a sidechannel is generated and transmitted, wherein the main channel and theside channel may be decoded separately from each other and bothrepresent the information signal. The main channel and the side channelare generated, however, so that the side channel is more robust againsttransmission channel influences than the main channel. When a situationresults in which the SNR of the system is smaller than the necessitatedSNR for the main channel, i.e. when the transmission channel is suchthat the robustness of the main channel is no longer sufficient,according to the invention a switchover to the side channel is executedwhich is more robust against transmission channel influences and maythus still be decoded, while the main channel may no longer be decoded.The connection from the receiver to the transmitter will thus not simplybe interrupted, but the reproduction will be continued using the data ofthe side channel which are robust against the transmission channelinfluences, although of course no main channel is reproduced any more.As the main channel and the side channel represent the same information,information is still provided to the radio broadcast listener and/or thebroadcast viewer.

In embodiments of the present invention it will be the case, however,that the side channel is a substantially more compressed representationof the information than the main channel. In particular in the contextof a lossy data compression, thus the reproduction quality in thereproduction of the side channel will be worse than the reproductionquality when the main channel can be reproduced. This is not asproblematic from the point of view of the radio broadcasting subscriber,however, as every radio broadcasting subscriber prefers a reproductionwith reduced quality to no reproduction at all.

This embodiment is advantageous in particular in so far as the overalldata rate for transmitting the main channel and the side channel is notsubstantially higher than when only the main channel is transmitted, asfor encoding a very lossy compressed data signal inherently less bitsare needed than for encoding a higher-quality data signal. It is inparticular advantageous that the bits necessitated for the side channelare at most half as many as those bits necessitated for the main channeland in particular even less than a tenth of the bits necessitated forthe main channel.

In a further embodiment, the main channel and the side channel are nottransmitted synchronously to each other, but with a time shift so thatthe main channel is delayed with regard to the side channel. On thereceiver side, the side channel which arrives at the receiverearlier—with regard to a certain point in time of the originalinformation data—is buffered into the receiver while no or only alimited buffering of the main channel is necessitated. This means thatwhen the transmission channel situation is favorable, the main channelcan be transmitted without delay and also switching over from the mainchannel to another main channel is possible without problems. Theoccurrence of a switching delay would only be perceivable if atransmission channel degradation took place exactly at the time ofswitching, with the consequence that the side channel would have to beused.

The delay of the main channel as compared to the side channel is furtheradvantageous to the extent that, when a transmission failure occurswhich is so severe that not even the side channel is received withsufficient quality, the side channel can be reproduced at least for thetime period for which the side channel was stored. In embodiments, astorage time of more than 10 seconds and, in particular, of even morethan 20 seconds, and advantageously more than 30 seconds, is selected,which, while leading to a certain storage requirement, is not criticalwith regard to the favorable availability of large memories. Thedecisive advantage is, however, that the radio broadcast receiver mayreplay for 30 seconds solely from its own memory—although of lowerquality—without the same receiving valid signals. The fact that the sidechannel needs less bits than the main channel simultaneously eases thenecessitated memory requirement in the receiver, which a user will beglad to accept, as the reception of the side channel is already anemergency situation which does not correspond to a normal situation but,when the side channel guarantees intelligibility of speech, isparticularly agreeable for the user when the user is listening to aninteresting radio program in which the spoken word is particularlyimportant and its intelligibility is guaranteed by the side channel.

Other features, elements, steps, characteristics and advantages of thepresent invention will become more apparent from the following detaileddescription of preferred embodiments of the present invention withreference to the attached drawings

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, embodiments of the present invention are explained inmore detail with reference to the accompanying drawings, in which:

FIG. 1 shows a block diagram of an embodiment of a device for generatinga signal to be transmitted;

FIG. 2 shows different embodiments of the encoder of FIG. 1;

FIG. 3 shows an alternative embodiment of the encoder of FIG. 1;

FIG. 4 shows a special implementation of the source encoder of FIG. 2;

FIG. 5 shows a device for generating a decoded signal; and

FIG. 6 shows a special embodiment of the device of FIG. 5;

FIG. 7 shows a flowchart of the steps executed by the radio broadcastingsubscriber, e.g. in a digital receiver.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, a so-called “graceful degradation” isachieved, i.e. a radio broadcasting reproduction of audio and/or videodata below the threshold needed for the main channel. Thus, the sidechannel, e.g., contains the data of a low bit rate coder having the samecontent as the main channel. It is the purpose to generate a substitutesignal when failures occur which may be implemented, e.g. based on thetechnology of generating comfort noise in speech encoders, i.e. usingdata transmitted in the side channel. This data is used in thereceiver-side signal synthesis, in order not to leave signal synthesisonly with noise generation but to control the signal synthesis at leastin such a detailed way that speech intelligibility is, for example,produced in such a way that e.g. news can still be understood.

Further, in one embodiment, a time shift between the main channel andthe side channel is executed, so that the main channel is delayed withregard to the side channel. According to the invention, further a morerobust modulation is executed for the side channel than for the mainchannel with the main audio data.

A modulus in DRM technology for the main channel, which is referred tothere as a media service channel (MSC), has a bit rate of 14.5 kb/s, forexample. In the side channel, in this embodiment additional data aretransmitted with 1 kb/s in the SDC (service description channel) in amore robust way and, e.g., encoded with an advance of 30 seconds. Themain channel is, therefore, delayed by 30 seconds as compared to theadditional channel. When the connection is interrupted for up to 30seconds, the side channel is reproduced. If the SNR is reduced below thethreshold of the MSC, but above the threshold of the SDC, the sidechannel is also reproduced. The invention may thus be used in atransmission system for digital media transmission in which the mainchannel exists which comprises main media data and in which further theside channel exists in which the same media data is represented with ahigher compression and a lower bit rate, wherein the side channel has amore robust modulation than the main channel, which necessitates a lowerminimal SNR for the transmission and the encoding of the side channel ascompared to the main channel. Depending on the implementation, the mainchannel may be delayed in time with regard to the side channel.

In the following, detailed reference is made to FIGS. 1 to 7. FIG. 1shows a device for generating a signal to be transmitted which isfinally output by a transmit antenna 10 which receives its transmitsignal from a transmitter 12. An encoder 14 is connected upstream fromthe transmitter 12 which receives information 16 as input data whichinclude audio and/or video information, and which provides an outputsignal to the transmitter 12 including both main-channel data 18 a andalso side-channel data 18 b. Depending on the implementation of theencoder 14 and the transmitter 12, the data 18 a, 18 b is transmittedbetween the encoder 14 and the transmitter 12 in two separate datastreams, or the data is multiplexed or transmitted to the transmitteralready after a carrier modulation in a data stream, wherein thetransmitter then only has the task, as is performed by a typical HFfront end, i.e. to mix and amplify the base data signal onto an HFcarrier.

In particular, the encoder is implemented to generate the main channel18 a and the side channel 18 b which may be decoded separate from eachother and thus both represent the information signal.

To obtain a decoded version of the information, according to theinvention, either the side channel or the main channel is sufficient. Inother words, the data of the side channel alone without the data of themain channel is sufficient to provide at least a lower-qualityrepresentation of the information on the encoder side.

According to the invention, the encoder 14 is further implemented togenerate the main channel 18 a and the side channel 18 b such that theside channel is more robust against transmission channel influences thanthe main channel. This robustness may be generated in different ways,for example by the use of a more robust modulation for the side-channeldata or by introducing a higher redundancy into the side-channel datathan into the main-channel data.

FIG. 2 shows a detailed illustration of the encoder 14 according todifferent embodiments of the present invention which are optionallyillustrated in FIG. 2. The encoder 14 may include a source encoder 20, aredundancy encoder 22, a mapper 24 and/or a modulator 26. Information 16is fed into the source encoder 20. Depending on the implementation, thesource encoder 20 already provides the side channel 18 b and the mainchannel 18 a, as is illustrated by the continuous lines between thesource encoder 20 and the redundancy encoder 22. Alternatively, however,the source encoder may be implemented such that it only provides onesingle version which is then, as is illustrated by the dashed linebetween the block 20 and the block 22, fed into the redundancy encoder22, which then, for example by different redundancy encoding which isreflected by different code rates, generates the side channel 18 b andthe main channel 18 a on the output side, as is illustrated by thecontinuous lines between block 22 and block 24. Alternatively, theredundancy encoder might also be implemented to generate only one singleredundancy-encoded data stream on the output side which, as isillustrated by the dashed line between block 22 and block 24, is fedinto the mapper 24 which than generates the main channel 18 a and theside channel 18 b using different mapping rules.

Alternatively, the mapper 24 may also generate one single output datastream and supply the same to the modulator 26, which, e.g., executes anFDMA, TDMA or CDMA modulation method, i.e. one of the known frequencymultiplex, time multiplex or code multiplex methods or a combination ofthose methods, as it is known in the art. Depending on theimplementation, the modulator may provide one single signal on theoutput side, which would then include both the main channel and also theside channel already in one single data stream, or the modulator mayprovide the side channel and the main channel as separate data streamswhich are then combined with each other in the transmitter 12 before onesingle antenna signal is emitted which includes both channels.

In principle, it is sufficient that in one of blocks 20, 22, 24, 26different channels with different robustnesses are generated. However,also different robustnesses may be cumulated. Thus, for example the sidechannel generated by the redundancy encoder 22, which is per se alreadymore error-resistant due to the greater redundancy, may additionally besubjected to a mapping rule in the mapper which is more robust thananother mapping rule with which the main channel may be provided,wherein additionally in the modulator for the side channel a moreerror-resistant modulation method may be used than for the main channel.

According to the invention it is advantageous, however, for the sourceencoder 20 to generate two different output data streams, wherein theside channel has a low bit rate and the main channel has a high bit ratewhich may then be processed by a combination of blocks 22, 24, 26, orwhich, e.g., only receive a different robustness by the redundancyencoder 22, wherein the redundancy encoder then already combines bothdata streams according to a certain regulation on the output side sothat only one data stream enters the mapper 24 and one data stream exitsthe same. Alternatively, however, the redundancy encoder may simplyprocess both input-side data streams with the same code rate, whereinthe mapper or the modulator would then generate the more differentrobustness of the two data channels.

Different robustnesses may be achieved in the redundancy encoderaccording to the invention, e.g. by using a Reed-Solomon code or an FECcode, e.g. with a feedback shift registers, comprising a certaingenerator polynomial and operating with or without puncturing. The coderate describes the number of output bits for a certain number of inputbits and is smaller than 1 due to the redundancy adding. For the sidechannel a code rate smaller than 0.5 may be used, while for the mainchannel a code rate higher than or equal to 0.5 may be used.

With regard to the mapper 24, for the main channel and the side channeldifferent mapping rules may be used. A mapping rule has a certain numberof symbols in the complex plane, wherein for a QPSK mapping only foursymbols exist in the complex plane, while for a 16-QAM mapping forexample 16 symbols exist in the complex plane. This means that a decoderwith QPSK only has to differentiate between four different symbols,while a decoder with 16 QAM already has to differentiate between 16different symbols. The minimal SNR for a QPSK mapping is thussubstantially lower than the minimal SNR for a 16-QAM mapping. However,for every modulation symbol in a 16-QAM mapping four data bits aretransmitted, while with OPSK for each symbol only two data bits have tobe transmitted. Mapping has a great influence on robustness againsttransmission channel influences.

Alternative modulation methods may also be used, like, for example, DPSKor 8 QAM. Also hierarchical modulation methods in which, e.g., a 16 QAMis overlaid onto a QPSK, may be implemented for the different channels.Thus, also for the main channel 64 QAM may be used, and for the sidechannel 16 QAM may be used.

Also in the modulator 26 different robustnesses may be generated, when,e.g., for a CDMA modulation code sequences of a different length areused for the side channel and/or for the main channel, or when in anFDMA modulation for the different channels different frequencybandwidths are used, or in a CDMA modulation time slots of a differentlength are used.

In the following, reference is made to implementations of the sourceencoder 20 of FIG. 2. Depending on the implementation, the sourceencoder of FIG. 2 may include an audio encoder 30 and a speech encoder32, connected in parallel. A delay is connected between the audioencoder 30 and a combiner, like, e.g., a multiplexer 34, wherein thedelay means is designated by 36. It may, e.g., be implemented as an FIFOmemory which is dimensioned such that more than 10 seconds,advantageously more than 20 seconds and in particular more than 30second of data is stored. The delay means 36 thus feeds the MSC input ofthe combiner 34, while the speech encoder 32 in this implementationfeeds the SDC input of the combiner. Of course, also additional delaysmay exist both in the main channel and also in the side channel 18 a, 18b, as long as the delays in the main channel are greater than the delaysin the side channel. In particular, the data rate in the MSC, which isthe media service channel, is 14.5 kb/s, while the data rate in the sidechannel, which is the SDC and/or the service description channel, is at1 kb/s.

Other data rates are also possible, wherein in particular ratios betweenthe MSC and the SCD and/or between the main channel and the side channelof <2, in particular <5 and again in particular <10 are advantageous.

In the embodiment illustrated in FIG. 3, thus the encoder 8 whichgenerates the main channel 18 a is an audio encoder and is implementedseparately from the encoder 32 which generates the side channel 18 b andis only implemented as a speech encoder. The speech encoder 32 mayprovide complete speech encoder frames. It may, however, alternativelyalso only output coefficients as a side channel serving for describingthe spectral envelope. In particular, the speech encoder 32 will beimplemented such that the coefficients for describing the spectralenvelope are quantized so finely and are transmitted so often that onthe receiver side using this information a speech intelligibility isachieved. If the speech encoder is implemented as an LPC encoder it isadvantageous to transmit the LPC coefficients which were calculated bythe speech encoder or it is advantageous to transmit coefficientsderived from the LPC coefficients (LPC=linear predictive coding).Coefficients which are derived from LPC coefficients are, for example,quantized or differentially encoded (delta-encoded) coefficients, as itis indicated at 40 and 42 in FIG. 4.

Alternatively, the source encoder 20 of FIG. 20 may also be implementedas it is illustrated in FIG. 4. Such an encoder is, for example, usedaccording to MPEG-4 or MPEG-1, Layer III (MP3). By a filter bank 41, theinformation signal 16 is converted into a spectral illustration suppliedto a quantizer 43. The filter bank 41 may here be a subband filter bank,e.g. having 16 or 32 filter bank channels, or may be an MDCT filterbank, e.g. having 512 coefficients or 1024 coefficients, wherein also anoverlap and add functionality for the time domain aliasing cancellation(TDAC) is to be used in a corresponding decoder.

The spectrum output by the filter bank 41 and/or the spectralillustration output by the filter bank 41 is quantized in the quantizer43. The quantizer 43 is controlled by a psychoacoustic model 44 which isimplemented to calculate the psychoacoustic masking threshold for eachband and to make the quantization so coarse that the quantization noiseis below the masking threshold. The quantized spectral values output bythe quantizer 43 are supplied to a Huffman encoder 45. It is noted thatthe quantizer 43 not only calculates quantized spectral values but alsoscale factors which represent the spectral coarse structure of thespectral illustration. In contrast, the spectral fine structure iscontained in the quantized spectral values.

For Huffman encoding, the Huffman encoder 45 uses a plurality ofpredefined code books, wherein according to the MPEG-AAC standard twelvedifferent code tables are used which are all different in the valuerange of the elements and/or spectral values or groups of spectralvalues encoded by the code table. Every code table is identified by itscode table number, which is, just like the scale factors, supplied to abit stream formatter 46 and necessitated on the decoder side to executea decoding using the correct code table.

The output data stream generated by the bit stream formatter 46 thenrepresents the main channel, while the side channel is generated using aside channel selector 47. The side channel selector is implemented toselect a certain portion of the data coming into the main channel tooccupy the side channel using these data. The less data is selected, thelower the data rate will be in the side channel, which is desirable forreasons of a responsible handling of the transmission bandwidth.However, a certain minimum measure of data is needed not only togenerate a pink noise on the receiver side but to be better, for exampleto cause speech intelligibility. For this purpose, scale factors and/orcode table numbers are supplied depending on the suitability andnecessity of a delta encoding 40 and a subsequent Huffman encoding 42.For the code table numbers, a delta encoding will not be as suitable.However, by a delta encoding of scale factors a further redundancyreduction may be achieved. The quantized spectral values are nottransmitted in the side channel. I.e., the spectral fine structure isnot transmitted in the side channel. Here, only the spectral coarsestructure exists.

Depending on the implementation, the side-channel data may thus comefrom a low-rate speech encoder or from a low-rate audio encoder. Thus,even a part of or also all coefficients of the side channel may comefrom coefficients of the encoder of the main channel. In particular,when the main encoder is a subband-based audio encoder, it isadvantageous to select the coefficients of the encoder of the mainchannel, representing the scale factors, into the side channel.Depending on the implementation, the indices for Huffman code books mayalso be used as selection data.

It is in particular to be noted, as is also indicated in FIG. 1, thatfurther side channels 18 c may be used which may be equipped withdifferent time offsets and/or different robustnesses for transmission.

FIG. 5 shows a special implementation of a device for generating adecoded signal according to an embodiment. The device illustrated inFIG. 5 includes a receiver stage 50 which may be coupled, e.g., to areceive antenna which is not illustrated in FIG. 5. Then, the receiverstage 50 contains a typical receiver front end, for example with adownmixer with a coupled local oscillator, to downmix the transmittedspectrum into the base band and/or into an intermediate frequency band.

The receive signal which is received by the receiver stage 50 includes amain channel and a side channel which may be decoded separately fromeach other. In particular, a minimal signal/noise ratio which is neededfor decoding the side channel is smaller than a minimal signal/noiseratio which is needed for decoding the main channel. I.e., the sidechannel is more robust against transmission characteristics of thetransmission channel than the main channel.

Downstream from the receiver, depending on the implementation, a channelseparation stage 51 is connected to separate the side channel from themain channel already on the HF side. Depending on the implementation,this functionality may, however, also be integrated in a decoder 52which is directly coupled to the receiver and which generates the mainchannel separate from the side channel. The inventive broadcastingsubscriber device, as is illustrated in FIG. 5, further includes aquality observer 53 which is implemented to assess the receive quality.Here, a quality observer may fall back on different signalsschematically indicated by continuous and/or dashed lines in FIG. 5. Thequality observer 53 may observe the main channel before decoding and/orbefore a delay possibly used there or after a delay possibly used there.The quality observer 53 may, however, alternatively or additionally,also use output data of the decoder and/or certain intermediate datawhich arise in decoding and/or decoder output information. If thedecoder for example determines in the Huffman decoding that a certainnumber of invalid code words exists, this already indicates a badreceive quality which is reported to the quality observation and/ormonitoring means 53.

The quality observation means 53 is implemented to provide a switchoversignal when a receive quality is determined which is less than a receivequality source, wherein the signal is, in the most general case,supplied to the decoder 52 which may then switch over from areproduction of the main channel to a reproduction of the side channel.

The changeover switch which is, e.g., contained in the decoder 52 orwhich may also be implemented separately may thus be controlled by thequality observer to provide the side channel as the decoded signal whenthe receive quality is less than a threshold quality, and to provide themain channel as a decoded signal when the receive quality is greaterthan or equal to the threshold quality.

In the following, with reference to FIG. 6, an alternative embodiment ofthe inventive receiver is illustrated. The functionality of the elements51, 52, 53 is implemented in FIG. 6 by alternative and/or by additionalelements, while the delay 54 of FIG. 5 which is used to delay the sidechannel with regard to the main channel, i.e. to compensate theencoder-side delay, is not illustrated in FIG. 6. It is to be noted thatthe delay may be built in between any blocks in order to guarantee that,in case a complete breakdown occurs, at least according to the datastored in the delay means a data output may be executed solely using thedata already stored within the decoder.

In FIG. 6, a demodulator 60 is connected downstream to the receiverstage 50 of FIG. 5, wherein the demodulator may execute a demodulationof the underlying modulation method, like, e.g., of a TDMA, FDMA or CDMAmethod. Hereupon, depending on the implementation, a demapper 61 may beconnected downstream, which will typically operate using softinformation to back-map the modulation symbols into bits. The bitsrepresented by soft information are supplied to a channel decoder 62which may, for example, be implemented as a Viterbi decoder or as aReed-Solomon decoder. The channel decoder 62 is based on the fact thaton the decoder side a redundancy was introduced by the redundancyencoder 22 which is used by the channel decoder for purposes of improvedreceive quality and/or reduced bit error rate on the decoder side. Theoutput data of the channel decoder are supplied to a source decoder 63which will be the counterpart of the encoder side source encoder andwhich, in particular if the encoder of FIG. 4 is considered, willcomprise first a bit stream deformatter, a downstream Huffman decoder,again a downstream re-quantizer and finally a synthesis filter bank togenerate a decoded audio signal.

If the source decoder 63 receives main-channel data, it comprises alldata needed for decoding including the spectral fine structure, and ahigh-quality output is achieved. If, however, only side-channel data istransmitted, like, for example, the spectral envelope of the originalinformation signal, the source decoder will, for example, execute asignal synthesis, wherein the spectral fine structure is synthesized andweighted using the transmitted data of the spectral coarse structure,such that a synthesized spectrum is generated which is then supplied tothe synthesis filter bank to generate a decoded audio signal which atleast comprises speech intelligibility. The same procedure may be usedwhen scale factors and/or code table indices are transmitted. In thiscase, all transmitted data are used according to their original purpose,while non-transmitted data are synthesized, for example by the syntheticgeneration of spectral values which are, regarded together in a scalefactor band, weighted such that a certain energy distribution isachieved in the band, wherein the absolute energy is basicallydetermined anyway by the scale factor which was directly transmitted inthe side-channel data stream.

Alternatively, the side-channel data stream may also be a band-limitedrepresentation (e.g. up to 4 kHz) of the original data, so that the sidechannel and the main channel are only different regarding theirbandwidth. In this case, the decoder for the side-channel data would notsynthesize any further spectral values, but would only generate thenarrow-banded signal the way it is.

If the side-channel data stream is the output signal of a speechencoder, however, like, for example that of a CELP encoder, as it isused in GSM, the inventive broadcast/radio receiver will also include aGSM speech decoder to generate the side-channel data when the level ofthe receive quality threshold is fallen below.

The quality observer 53 in FIG. 6 may be supplied with information aboutreceive quality from different locations in the processing chain. Thequality observer 53 is, however, advantageously fed directly by achannel estimator 64 typically present anyway in a radio receiver,wherein the estimator is implemented to estimate the wirelesstransmission channel with or without pilot tones. Depending on theimplementation of the channel estimator, the same, e.g., alreadyprovides an SNR which is then only to be compared with the threshold bythe quality observer 53 in order to generate a control signal 65 whichcontrols a switch 66 to either output the main channel or the sidechannel.

In the illustration illustrated in FIG. 6 it was assumed that the sourcedecoder outputs the main channel and the side channel in parallel, i.e.already with a compensated delay. This has the advantage that when thereceive quality is fallen below a switchover has to be performed in theswitch 66, i.e. that the decoded side-channel signal already exists.Alternatively, however, for saving current in particular in mobiledevices, only the main channel can be separated out to be decoded,wherein a decoding of the side channel is only started when the receivequality falls below the threshold. This has the advantage that at theoutput of the source decoder only one signal is applied and that, thus,for the second signal, i.e., for example, for the main channel in theextreme operation or for the side channel in the normal operation noprocessor resources or battery resources are necessitated. In so far,the switch 66 also has to be regarded schematically, as it may alreadybe integrated into the functionality of the decoder and may in principlealso exist in any location where the main channel and the side channelalready exist separately. If, e.g., the main channel and the sidechannel are already applied at the output of the demapper 61, the(changeover) switch 66 may already be arranged there to either feed themain channel or the side channel into the downstream channel decoder.

With regard to the quality observation 53 it is to be noted that, whenthe channel estimator 64 is not accessed or when additional data on thereceive quality are desired, the main channel or the combinedmain-channel/side-channel signal may be accessed in any location toobtain an impression on the current receive quality and in particular onthe receive quality of the main channel.

In the following, with reference to FIG. 7, a principle procedureaccording to an embodiment of the present invention on the receive sideis demonstrated. It is assumed that a reproduction of the main channelis currently going on, as is indicated in step 70. In parallel to thereproduction of the main channel in step 70, an assessment of thequality of the main channel is taking place in step 72. Alternatively oradditionally, as it was illustrated with regard to FIG. 6, also thereceive quality in general, i.e. without direct relation to the mainchannel, may be assessed, for example when the channel estimatoroperating with or without pilot tones is used.

Only for reasons of clarity, in FIG. 7 reference is made to a “qualityof the main channel”, which is assessed. If the quality assessed inblock 72 is above the threshold, the reproduction of the main channel iscontinued in step 70. If it is determined, however, that the qualitylies below the threshold, a switchover to the side channel is executedas illustrated in step 74. Hereupon, the side channel is reproduced, asit is indicated in step 76. In parallel or subsequently, an assessmentof the quality of the main channel or generally of the receive qualityis executed, as is illustrated at 78, wherein step 78 may be implementedidentically to step 72. Differences are only that here a quality isassessed which is not the quality of the currently reproduced channel.If it is determined in field 78 that the quality is below the threshold,the reproduction of the side channel is continued. If it is determinedin step 78 that the quality is above the threshold, a switchover to themain channel is executed, as illustrated in step 79.

When the signal is a video signal, the side channel may be a downsampledversion of the main channel. The decimation of data is executed by thespatial decimation of, e.g., every second pixel per image vertically andhorizontally and/or by the time decimation of, e.g., every second imageof a sequence or by any other decimation measures.

Depending on the circumstances, the inventive method may be implementedin hardware or in software. The implementation may be on a digitalstorage medium, in particular a floppy disc or a DC havingelectronically readable control signals which may cooperate with aprogrammable computer system so that the method is executed. In general,the invention thus also consists in a computer program product having aprogram code stored on a machine-readable carrier for executing theinventive method when the computer program product runs on a computer.In other words, the invention may thus be realized as a computer programhaving a program code for executing the method, when the computerprogram runs on a computer.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which fall withinthe scope of this invention. It should also be noted that there are manyalternative ways of implementing the methods and compositions of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutationsand equivalents as fall within the true spirit and scope of the presentinvention.

The invention claimed is:
 1. A device for generating a decoded signal,comprising: a receiver configured to receive a receive signal comprisinga main channel and a side channel which are capable of being decodedseparate from each other, wherein a minimal signal/noise ratio used todecode the side channel is smaller than a minimal signal/noise ratioused to decode the main channel; a decoder configured to generate a mainchannel which is separate from the side channel; a quality observerconfigured to assess a receive quality; and a changeover switch which iscontrollable by the quality observer to provide the side channel as adecoded signal when the receive quality is lower than a thresholdquality and to provide the main channel as a decoded signal when thereceive quality is higher than or equal to the threshold quality.
 2. Thedevice according to claim 1, further comprising a channel estimatorconfigured to estimate a transmission channel with or without pilotsymbols, and wherein the channel estimator is implemented to feed thequality observer with channel data.
 3. The decoder according to claim 1,further comprising a demodulator, wherein the quality observer isimplemented to assess data before and after the demodulator for qualityobservation.
 4. The device according to claim 1, wherein a demapper isfurther implemented and wherein the quality observer is implemented toevaluate an output signal of the demapper for quality observation. 5.The device according to claim 1, wherein the decoder further comprises achannel decoder and wherein the quality observer is implemented to usethe channel decoder output data for quality observation.
 6. The deviceaccording to claim 1, wherein the quality observer is implemented toexecute quality observation using the main channel.
 7. The deviceaccording to claim 1, wherein the decoder comprises a source decoderwhich is implemented to detect invalid code words representing quantizedvalues, and wherein the quality observer is implemented to execute aquality observation based on a number of detected invalid code words. 8.A method for generating a decoded signal, comprising: receiving, by areceiver, a receive signal comprising a main channel and a side channelwhich are capable of being decoded separate from each other, wherein aminimal signal/noise ratio used to decode the side channel is smallerthan a minimal signal/noise ratio used to decode the main channel;generating, by a decoder, a main channel separate from the side channel;assessing, by a quality observer, a receive quality; and providing, by achangeover switch, as a decoded signal, the side channel, when thereceive quality is lower than a threshold quality, or providing, as adecoded signal, the main channel, when the receive quality is higherthan or equal to the threshold quality, wherein at least one of thereceiver, the decoder, the quality observer, and the changeover switchcomprises a hardware implementation.
 9. A non-transitory storage mediumhaving stored thereon a computer program comprising a program code forexecuting, when the computer program runs on a computer, a method ofgenerating a decoded signal, the method comprising: receiving a receivesignal comprising a main channel and a side channel which are capable ofbeing decoded separate from each other, wherein a minimal signal/noiseratio used to decode the side channel is smaller than a minimalsignal/noise ratio used to decode the main channel; generating a mainchannel separate from the side channel; assessing a receive quality; andproviding, as a decoded signal, the side channel, when the receivequality is lower than a threshold quality, or providing, as a decodedsignal, the main channel, when the receive quality is higher than orequal to the threshold quality.